1. Field of the Invention
This invention relates to a torque control apparatus for an engine and an automatic transmission and, more particularly, to a control apparatus in which shocks produced during a gear shift are moderated by a torque reduction control for the output of the engine during the gear shift.
Generally, an automatic transmission for an automotive vehicle is equipped with a torque converter and a transmission gear mechanism. The torque converter transmits torque from an output shaft of the engine to a turbine shaft by changing its speed of rotation. The transmission gear mechanism transmits the torque of the turbine shaft to driving wheels after further changing the rotational speed and, when the automotive vehicle is propelled backward, by reversing the rotational direction.
The transmission gear mechanism ordinarily includes a planetary gear system having gears, a plurality of clutches for locking or unlocking a turbine shaft and predetermined gears, and a plurality of brakes for fixing or releasing predetermined gears. In the transmission gear mechanism, turn-on and turn-off patterns for the clutches and brakes are changed by a hydraulic mechanism, so that a power transmission route in the planetary gear system is switched. It is, therefore, possible to provide gear shifts and to switch between forward and backward driving.
2. Description of Related Art
During a gear shift in such an automatic transmission, such as a downshift from third gear to second gear, a 2-4 brake, for example, is turned on or engaged, and a 3-4 clutch and coast clutch, for example, are turned off or disengaged. However, since the amount of torque transmission abruptly changes midway through such a gear shift, shocks due to gear shifts can occur and be a problem.
To prevent the occurrence of such shocks during a gear shift, torque control means or apparatuses, by which the output torque of the engine is temporarily reduced during the shift, are ordinarily provided in automatic transmissions. More specifically, in an automatic transmission, for example, after detecting a start time of a gear shift operation on the basis of a variation ratio of an input speed of the automatic transmission, the torque of the engine is reduced only during a predetermined period from the start time to a time counted up by a timer, as described in Japanese Unexamined Patent Publication No. 60-248,445.
However, in such a known automatic transmission, time required for gear shifts and timings or periods in which shocks tend to occur during the gear shifts have not been set so that torque is reduced. This causes problems in that an increasing speed of the engine and, therefore, vehicle acceleration may deteriorate due to overreduction of torque after an end of a gear shift. Also, shocks produced during gear shifting may occur due to an early release of torque reduction.
Similarly, as is clear from Japanese Unexamined Patent Publication No. 60-227,049, a known control apparatus performs a reduction control of the output of the engine between starting and terminating speeds. Such a reduction control is performed by detecting the engine speed, determining the starting speed of the reduction control at a time at which the variation ratio of the engine speed is equal to or larger than a predetermined value during an upshift of the automatic transmission, and predicting the terminating speed of the reduction control of the output torque, from a memorized table of data, on the basis of the starting speed in accordance with the degree to which the throttle valve is open, vehicle speed, and the kinds of gear shift, such as a shift from the 1-speed gear to the 2-speed gear.
In such a control apparatus, since the reduction control of the output torque is started from the time at which the variation ratio of the engine speed becomes equal to or larger than the predetermined value, the reduction control is carried out during the gear shift surely and without a malfunction.
However, when the variation ratio of the engine speed becomes equal to or larger than the predetermined value, which may occur for example, when the engine speed becomes equal to or larger than the predetermined value in a short or relatively long time, the engine variation conditions may be different, even if other conditions right before the gear shift, for example, the vehicle speed, are the same. Consequently, although the speeds at which the reduction control of the output torque should be terminated differ according to differences in these conditions, since the terminating speed is determined as a unitary value memorized in the table, termination of the reduction control of the output torque can not be performed precisely. As a result, if the reduction control of the output of the engine is terminated early, shocks from the gear shift are not effectively moderated. Also, if the reduction control is continued more than necessary, deterioration of acceleration performance of the vehicle becomes a problem.
In particular, during a gear shift which involves a 3-4 clutch having a drift on ball, for example, during a downshift from third gear to second gear, the operational characteristics of the drift on ball is varied in response to a driving condition. Consequently, a period required for the gear shift varies, and disadvantages such as those mentioned above arise.
Shocks due to a gear shift are generally determined according to a torque input into the transmission from the engine, i.e., an input torque of the transmission. Therefore, it is desirable to change torque reduction characteristics in accordance with the input torque, for instance, to change a timing for reducing the torque. Another previously known torque control apparatus shifts the timing for reducing the torque in response to the engine speed, which relates to the output torque thereof. Such an apparatus is described in Japanese Patent Publication No. 2-20,817.
Even though the speed of the engine generally relates to the output torque of the engine, the speed of the engine does not precisely relate to the output torque of the engine, much less the input torque for the transmission. Thus, a torque control apparatus in which the timing for reducing the torque is shifted on the basis of the speed of the engine does not always start the reduction of torque at an appropriate timing determined in accordance with the occurrence of torque on the side of the transmission during a gear shift. Accordingly, it is impossible to effectively prevent the occurrence of shocks due to the gear shift.
Moreover, during a gear shift in an automotive vehicle equipped with, for example, a four speed gear shift mechanism, transmitting torque is temporarily absorbed in the transmission gear mechanism by operation of elements such as clutches right after a shift starts. Consequently, a phenomenon in which acceleration in a lengthwise direction of the vehicle body falls down, i.e., a so called "pull-down" phenomenon, can occur. After the middle of the gear shift period, when the speed of the engine begins to be lowered, since the amount of engine torque transmitted to driving wheels is increased quickly, a phenomenon in which acceleration in the lengthwise direction of the vehicle body increases quickly, i.e., a so called "push-up" phenomenon, can occur. A "push-up" shock, therefore, can be produced.
In order to prevent such a push-up shock, an automotive vehicle equipped with an automatic transmission, in which the torque of the engine is reduced during shifting, has been proposed. Such a vehicle is described in, for instance, Japanese Unexamined Patent Publication No. 60-227,049 mentioned above. In this automotive vehicle, if the reduction of the torque and the occurrence of the pull-down phenomenon are synchronized, a drop of acceleration in the lengthwise direction is easily produced. This causes shocks and, especially, pull-down shocks.
In one other previously known automotive vehicle, a timing at which torque begins to reduce is retarded until a timing at which the pull-down phenomenon is over or the speed of the turbine begins to change due to the shifting operation. However, although the automotive vehicle is able to prevent deterioration of the pull-down phenomenon and the occurrence of a pull-down shock, it is still impossible to suppress push-up shock sufficiently.